1. Field of the Invention
The present invention relates to a lithographic apparatus, an excimer laser and a device manufacturing method. This invention also relates to a device manufactured thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Between the reticle and the substrate is disposed a projection system for imaging the irradiated portion of the reticle onto the target portion of the substrate. The projection system includes components for directing, shaping or controlling the projection beam of radiation. The projection system may, for example, be a refractive optical system, or a reflective optical system, or a catadioptric optical system, respectively including refractive optical elements, reflective optical elements, and both refractive and reflective optical elements.
Generally, the projection system comprises a device to set the numerical aperture (commonly referred to as the “NA”) of the projection system. For example, an adjustable NA-diaphragm is provided in a pupil of the projection system.
An illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. The illumination system of the apparatus typically comprises adjustable optical elements for setting an outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of an intensity distribution upstream of the mask, in a pupil of the illumination system. A specific setting of σ-outer and σ-inner may be referred to hereinafter as an annular illumination mode. Controlling the spatial intensity distribution at a pupil plane of the illumination system can be done to improve the processing parameters when an image of the illuminated object is projected onto a substrate.
Microchip fabrication involves the control of tolerances of a space or a width between devices and interconnecting lines, or between features, and/or between elements of a feature such as, for example, two edges of a feature. In particular the control of space tolerance of the smallest of such spaces permitted in the fabrication of the device or IC layer is of importance. Said smallest space and/or smallest width is referred to as the critical dimension (“CD”).
With conventional projection lithographic techniques it is well known that an occurrence of a variance in CD for isolated features and dense features may limit the process latitude (i.e., the available depth of focus in combination with the allowed amount of residual error in the dose of exposure of irradiated target portions for a given tolerance on CD). This problem arises because features on the mask having the same nominal critical dimensions will print differently depending on their pitch on the mask (i.e., the separation between adjacent features) due to pitch dependent diffraction effects. Pitch is the sum of the feature width and the space between two subsequent features.
A difference in printed CD between two similar features such as lines arranged at two respective, different pitches, is referred to as an iso-dense bias or “TDB”. For example, a feature consisting of a line having a particular line width and arranged at a large pitch, will print differently from the same feature having the same line width and provided in a dense arrangement on the mask, i.e., arranged at a small pitch. Hence, when both dense and isolated features of critical dimension are to be printed simultaneously, a pitch dependent variation of printed CD is observed. Data describing a specific CD-pitch dependency are generally represented by a plot of CD versus pitch, referred to as a CD-pitch curve hereinafter. The phenomenon “iso-dense bias” is a particular problem in photolithographic techniques. Iso-dense bias is typically measured in nanometers and represents an important metric for practical characterization of lithography processes.
Generally, a mask pattern is designed in such a way that differences in dimensions of printed isolated and dense features are minimized to some degree, by applying a size bias to certain features. Applying, to the mask pattern, a size bias to certain features such as lines is referred to as feature-biasing and, in the case of lines, as line-biasing. The actual pitch dependency of printed CD depends, however, on the specific properties of the apparatus (such as aberrations and calibrations of the lithographic apparatus in use). Therefore, even in the presence of feature bias, a residual iso-dense bias may be present. Conventional lithographic apparatus do not directly address the problem of iso-dense bias. Conventionally, it is the responsibility of the users of conventional lithographic apparatus to attempt to compensate for the iso-dense bias by either changing the apparatus optical parameters, such as the NA of the projection lens or the σ-outer and σ-inner settings, or by designing the mask in such a way that differences in dimensions of printed isolated and dense features are minimized. However, such changes of machine settings may adversely affect the process latitude.
Generally, in a high volume manufacturing site different lithographic projection apparatus are to be used for the same lithographic manufacturing process step to ensure optimal exploitation of the machines, and consequently (in view of, for example, machine-to-machine differences) a variance and/or errors in CD may occur in the manufacturing process. Generally, the actual pitch dependency of such errors and the actual CD-pitch dependency depends on the specific layout of the pattern and the features, the aberration of the projection system of the lithographic apparatus in use, the properties of the radiation sensitive layer on the substrate, and the radiation beam properties such as illumination settings, and the exposure dose of radiation energy. Therefore, given a pattern to be provided by a patterning device, and to be printed using a specific lithographic projection apparatus including a specific radiation source, one can identify data relating to iso-dense bias which are characteristic for that process, when executed on that lithographic system. In a situation where different lithographic projection apparatus (of the same type and/or of different types) are to be used for the same lithographic manufacturing process step, there is a problem of mutually matching the corresponding different CD-pitch dependencies, such as to reduce CD variations occurring in the manufacturing process.
A known technique to match a CD-pitch dependency of a machine (for a process whereby an annular illumination mode is used) to a CD-pitch dependency of another machine is—in analogy with above described techniques to compensate an iso-dense bias—to change the σ-outer and σ-inner settings, while maintaining the difference between the σ-outer and σ-inner settings (i.e., whilst maintaining the annular ring width of the illumination mode) of one of the two machines. The nominal σ-settings are chosen so as to optimize the process latitude (in particular, the depth of focus and the exposure latitude). Therefore, this approach has the disadvantage that for the machine whereby the σ-settings are changed, the process latitude is becoming smaller and may become too small for practical use.
An actual pitch dependency as described above may be varying in time. For example, due to lens heating the aberration of the projection system may vary, and or due to heating and other instabilities properties such as illumination settings, and exposure dose of radiation energy may vary in time. Therefore there is the problem of controlling and keeping within tolerance a desired CD-pitch dependency.